The formation and properties of nanoparticles are areas of intense scientific research, having a wide variety of potential applications in biomedical, optical, electronic and structural material fields. Particles, with dimensions between 1-100 nanometers (nm) are typically identified as nanoparticles. The properties of nanoparticles fall in the range between those of bulk materials and atomic or molecular structures. While a bulk material typically exhibits constant physical properties regardless of its size, the properties of materials can change as the size of a material body approaches nanoscale. Examples of such size-dependent behavior include quantum confinement in semiconductor particles, surface plasmon resonance in some metal particles and superparamagnetism in magnetic materials. In recent years, the adjustable properties of nanoparticles have increasingly been exploited in many fields in order to engineer enhanced solutions and products.
One feature of nanoparticles is that they have a very high surface area to volume ratio. This can also provide a strong driving force for diffusion, especially at elevated temperatures. Sintering can therefore take place at lower temperatures, over shorter time scales than for larger particles. The large surface area to volume ratio can also reduce the incipient melting temperature of the material in nanoparticle form.
Nanoparticles may be used as discrete components, usually in a dispersing medium, or as precursors for forming larger bodies including thin films. In the latter case nanoparticles might be dispersed in a carrier and then coated onto a substrate as an ink. This coating can be dried, reacted and, or densified to form a desired film. The noted advantages of nanoparticles including enhanced diffusion and lower melting and sintering temperatures may be used advantageously in the fabrication of such engineered bodies including films.
A variety of methods have been demonstrated for forming nanoparticles including; attrition of macro materials, flame pyrolysis, plasma spraying, gas aggregation and precipitation methods.
In attrition, macro scale particles are ground in a ball mill, or other size reducing apparatus. The resulting particles are then classified to separate nanoparticles. Ball-milling is sometimes considered a “dirty” process because of a potential for contamination from the ball-milling components and processes. However, with the introduction of wear resistant components, e.g made of tungsten carbide, and better process control such impurities can be reduced to acceptable levels for many applications. Attrition methods commonly produce nanoparticles with a broad size distribution. Typical of a top-down approach, the average particle size decreases with longer processing time. For given manufacturing objectives, including cost or rate of production, the processing time required to achieve very small nanoparticles may become restrictive.
In flame pyrolysis, a liquid, gas, solution or mixed precursor is typically forced through an orifice at high pressure and burned in a combustible gas flame. The resulting product is classified to recover nanoparticles from by-product gases. Flame pyrolysis often produces aggregates rather than individual nanoparticles.
In plasma spraying nanoparticles are formed by injecting feedstock materials into the jet of a plasma torch where they can be evaporated and then quenched on exiting the plasma Plasma temperatures can approach 10000 K and a wide range of feedstocks including powders can be processed. The residence time of the material in the plasma is typically very short, so it is important that the starting feedstock dimensions are small enough to ensure complete evaporation. During processing, the plasma does not contact the electrodes, thus avoiding a possible source of contamination and permitting the use of a wide range of inert, reducing or oxidizing atmospheres. RF plasma methods have been used to make ceramic nanoparticles such as oxides, carbides and nitrides of Ti and Si.
Gas aggregation has been used to make nanoparticles of low melting point elements and in particular metals. Typically, the metals are melted and vaporized in a vacuum chamber and the vapor is fed into an inert gas stream where it is supercooled and condensed to form nanoparticles. These nanoparticles are entrained in the gas stream and may be collected or directly deposited therefrom.
Precipitation methods including sol-gel methods are also widely used and documented. Addressable compositions are restricted by the availability of suitable precursor materials.
A common limitation of the flame pyrolysis, plasma spraying and gas aggregation methods for nanoparticle formation is a difficulty in achieving compositional control which precludes the possibility of appropriately processing of compound nanoparticles which are comprised of multiple elements which exhibit significantly different tendencies to vaporize, or separate, during processing. This also limits their usefulness for applications where it is highly beneficial, or essential, to achieve precise compositional control of the nanoparticles.
In summary, the field of nanoparticle formation and application is still in an evolutionary phase. One limiting factor is the difficulty in forming nanoparticles of materials which exhibit a preferred loss of constituents during processing. New methods which can provide enhanced compositional control are highly desirable.
In the context of droplet spraying, U.S. patents, patent applications and other publication describing the design and application of nozzles for spraying and/or particle generation, including molten metal droplet spraying (U.S. Pat. No. 4,181,256 issued to Kasagi), solution droplet spraying (U.S. Patent Application No. 20060210640 issued to Kerkhof and U.S. Patent Application No. 20080041532 issued Chou et al.), vapor mixture spraying (U.S. Patent Application No. 20080226270 issued to Wendt et al.) and fuel injection (Roecker, R., “Spray Technology”, Brochure D03, SouthWest Research Institute, (1998), provide some background information on nozzles and spraying.
In the context of compound nanoparticles which will be addressable by method described herein and their applications, a method of employing nanoparticles of fully reacted compound semiconductor materials is described in commonly assigned, co-pending U.S. patent application Ser. No. 12/185,369, entitled “A Reacted Particle Deposition (RPD) Method for Forming a Compound Semi-Conductor Thin-film, filed Aug. 4, 2008. None of the references or techniques cited above provides a suitable method for generating such compound semiconductor nanoparticles.